This invention relates to so-called twin shaft, compulsory mixers. More specifically, a large volume twin shaft, compulsory mixer having a capacity exceeding 12 cubic yards is utilized in combination with an elevating concrete conveyor to mix and convey concrete from the foundation of a modular portable concrete plant. Problems related to discharge gate deflection and compulsory mixer chamber deformation are disclosed and solved.
Concrete mixers in North America are usually of the rotating tilting drum variety. In such mixers, a rotating cylinder that tilts on its axis of rotation is utilized. Initially, the drum is in a tilted orientation to have its open end elevated. Depending on the manufacturer, the drum is filled with the constituents of concrete including cement, aggregate, sand, and water either through its discharge opening or through the opposite end. Paddles are fastened to the interior of the rotating drum. Upon rotation, the constituents act against the paddles and the force of gravity to be stirred and moved within the rotating drum. In part, some of the mixing action is by the concrete being lifted by the mixer paddles then falling to the bottom of the drum into the rest of the concrete. By both interfering flows within the rotating drum and the paddle stirring against the force of gravity, mixing occurs.
Upon completion of mixing, drum tilting occurs about the axis of rotation to dispose the discharge end of the drum generally downward. To enhance discharge of the concrete, on some tilting drum mixer designs, the drum is provided with reverse rotation. During this reverse rotation, the now mixed concrete constituents are moved by the force of gravity interacting with paddles interior of the drum from the closed drum end towards the open drum end. The mixed concrete is discharged from the open end of the rotating and tilted drum.
Rotating drum mixers have their disadvantages. Mixing utilizing the force of gravity takes time. In the case of mixing low slump or optimum moisture materials their mixing efficiency is low. Further, in the usual cases, in order to permit discharge, the rotating drums must be elevated. This requires the elevated support of considerable weight. Further, since the drums are tilted after mixing occurs, considerable torque must be resisted. In the usual case, both foundation structures and upwardly extending structural supports must be supplied to such rotating drum mixers. Rotating drums are unsuitable for use as a foundation for other parts of a mixing plant.
So-called twin shaft xe2x80x9ccompulsory mixersxe2x80x9d for concrete are old and well known. These mixers, invented in 1888, cause the constituents of concrete to be rapidly mixed along interfering paths without rotating drums. Compulsory mixers have counter rotating paddle systems in an otherwise static mixing chamber to enable thorough mixing with great rapidity. In what follows, we will set forth the modem construction and usage of such mixers.
In their modern construction, compulsory mixers have an open top to a static mixing chamber. The static mixing chamber has a bottom defined from two horizontally disposed and interfering cylindrical shapes. A first cylindrical shape formed along a first horizontal axis defines a little over one half the volume and bottom profile of the static mixing chamber. A second cylindrical shape formed along a second horizontal and parallel axis defines a little over a second one half of the volume and remaining bottom profile of the static mixing chamber. The cylinders defining the bottom profile of the mixing chamber overlap or interfere at respective interfering sections interior of the volume of the mixing chamber. This interference occurs along cylindrical segments extending centrally of the volume of the static mixing chamber.
Counter rotating paddle systems effect mixing within such compulsory mixers. Each mixing paddle system rotates co-axially within and along the axis of the interfering cylinders defining the bottom of the chamber. A first paddle system has a first axis of rotation co-axial to the first horizontal axis of the first cylinder defining half the volume of the mixing chamber. A second paddle system has a second axis of rotation co-axial to the second horizontal axis of the second cylinder defining the remaining half of the volume of the mixing chamber. Each paddle system has canted paddles to sweep concrete constituents in their respective cylinders from the sides of the cylinders to and toward the interfering portion of the cylinders defining the volume of the static mixing chamber. Dual spiral motions directed to one static mixing chamber end occurs. During their rotation, the paddles systems overlap and interleave at the interfering portions of the cylinders defining the volume of the static mixing chamber.
The arrangement and rotation of each set of mixing paddles imparts to the concrete constituents a spiral pattern within each half of the volume of the static mixing chamber. The interfering portions of the cylindrical volumes defining the static mixing chamber result in the superimposition of the two spiral patterns. These superimposed and interupted spiral patterns produce a compulsory and interfering concrete constituent flow resulting in a three-dimensional interfering flow path within the static mixing chamber. A high degree of turbulence is promoted. Mixing at the interfering portions of the cylinders is most intensive, resulting in a rapid homogeneity and cement dispersion or thorough mixing of the concrete constituents.
Unlike the rotating drum mixer, the discharge of the mixed concrete constituents from a compulsory mixer does not use or require mixing chamber movement. Instead, it is necessary to supply the bottom of the static mixing chamber with an opening.
To discharge mixed concrete from the static mixing chamber, an elongate rectilinear opening is provided parallel to the axial length of the two cylinders defining the volume and bottom profile of the static mixing chamber. Specifically, at the juncture of the interfering cylinders along the bottom of the mixing chamber, there is placed an elongate rectilinear opening. This elongate rectilinear opening is opened and closed by a rotating gate.
The rotating gate is provided with a sealing surface that is correspondingly elongate and rectilinear with respect to the elongate rectilinear opening. In a first position, the rotating gate at the elongate rectilinear eccentric surface tightly seals the elongate rectilinear opening. When mixing occurs, concrete constituents, especially water, cement and sand, cannot easily escape out the bottom of the compulsory mixer chamber.
When mixing is complete and concrete discharge is desired, the gate is rotated. Rotation occurs from a position that seals the bottom of the chamber to a position that opens the bottom of the chamber. Discharge of the mixed concrete constituents from the interior of the static mixing chamber occurs.
It has been realized that rapid emptying of the mixed concrete is required to reduce mixing cycle times. For this reason, the opening of the rectilinear slot at the bottom of the mixing chamber must be maximized. In order to maximize this opening, the elongate rectilinear portion of the gate is eccentrically mounted with respect to the axis of rotation of the gate. Specifically, the gate defines a chord occupying about one third of the arc produced by the cylinder of rotation of the gate.
With such an eccentric gate, rotation of the gate through an arc of about 120xc2x0 is required. The top of the eccentrically mounted elongate rectilinear portion of the gate moves out of sealing relation to the rectilinear slot centrally of the static mixing chamber. As rotation continues, the sealing side of the eccentrically mounted elongate rectilinear portion of the gate is no longer disposed to the mixed concrete. Instead, the reverse side of the eccentrically mounted elongate rectilinear portion of the gate forms a mixed concrete discharge chute. This discharge chute forms flow path opening well over one-half of the cylinder of rotation defined by the rotating gate. Rapid discharge can occur.
During this described opening of the eccentrically mounted elongate rectilinear gate, the counter rotating paddle systems maintain their rotation. As a result, mixed concrete constituents are impelled to the open discharge gate. Rapid emptying of the compulsory mixer occurs not only responsive to the forces of gravity but additionally with respect to the sweeping action of the interfering paddle systems.
Modern concrete mixing plants, especially those mixing plants used for roads and runways, require mobility and production capacity. In addition to this, the selected mixer must uniformly mix the concrete without increasing the mixing time otherwise production capacity is diminished. Because of these shortcomings, the tilting drum mixer loses its utility. The tilting drum mixer is difficult to mount in its required elevated and torque reinforced disposition. Such mounting requires at least semi-permanent foundations. Moreover, mixing takes too long. Finally, such mixers cannot be used as foundations for the portable plants to which they are attached. Simply stated, a rotating drum is an unsuitable foundation for anything.
Compulsory mixersxe2x80x94because of their shorter mixing cycles and their ability to uniformly mix low-slump materialsxe2x80x94have found favorable use, especially in the European market. They have not been widely accepted in the North American paving market because such mixers have been constrained in batch capacity. Specifically, the largest compulsory mixers now manufactured in Europe are limited to batch sizes of about 4.5 to 6 cubic meters or 6 to 8 cubic yards of vibrated and compacted concrete. The largest compulsory mixer ever built is in the order of 7.5 m3 (9.9 cyd) of vibrated and compacted concrete. Generally, the European practice is to double batch or to load two batches in each hauling truck. This is practical in Europe because job production rates expected and customary are approximately half of expected and customary production rates in the North America. North American contractors need high production to be competitive. The required North American concrete production rates per hour could never be realized by following the accepted European practice of double batching. Thus the mixer batch sizes must match the full hauling ability of their trucks that varies from 7.5 to 12 cyd (sometimes 13 cyd) depending if they are are hauling on or off road.
Before this disclosure, compulsory mixers were mounted at an elevation where they generally overlie their required discharge. For example, where discharge occurs to a truck, the compulsory mixer is mounted at an elevation overlying the truck.
In an attempt to increase the capacity of compulsory mixer plants, and to hold a batch when a truck is not available, concrete discharging to a batching hopper has been utilized. In this case, the compulsory mixer requires even further elevation. First, elevation sufficient to discharge to the batching hopper occurs from the compulsory mixer. Thereafter, the batching hopper must be elevated to discharge to and to clear an underlying truck. Thus, the compulsory mixer must be at an elevation overlying both the batching hopper and the transporting truck.
Even where a batching hopper is utilized, mixing time in the compulsory mixer is nearly doubled. Simply stated, it takes almost twice as long to mix two batches in a compulsory mixer as it does to mix one large (combined) batch in a compulsory mixer. We have realized that the increase in size for a compulsory mixer would be extremely desirable for this type of mixer to gain acceptance in the North American market.
In U.S. patent application Ser. No. 09/255745, filed Feb. 23, 1999, entitled Portable and Modular Batching and Mixing Plant for Concrete, there is disclosed a compulsory mixer. As of the filing of this disclosure, publication of this application and design has not yet occurred.
In this disclosure, a so-called two-trailer portable and modular batch plant is disclosed. First, a mixer trailer includes a compulsory mixer, cement silo and a generator set. A second trailer is an aggregate trailer, control cabin and a water tank.
The compulsory mixer in this disclosure is placed on the ground, along with the trailer structure, so as to form the foundation for the plant. The required cement silo erects to overly the compulsory mixer. The compulsory mixer is unloaded at its rectilinear slot located centrally of and underneath the static mixing chamber by an underlying conveyor. The underlying conveyor receives, elevates and discharges the concrete either to a batching hopper or an awaiting truck.
The compulsory mixer in this disclosure is of limited capacity. It mixes about six cubic yards of vibrated and compacted concrete per batch. Consequently, the capacity of the plant is limited to under 300 cubic yards per hour (228 cubic meters per hour). If the hauling trucks are of 12 cyd (9.12 m3) capacity, then the truck must wait for two batches to be mixed and discharged before pulling away.
Aside from this disclosure, we are unaware of compulsory mixers unloading to an underlying conveyor. Accordingly, in this disclosure we claim novelty directed to a compulsory mixer unloading to an underlying conveyor. Such unloading by an underlying conveyor enables a compulsory mixer to serve as a foundation for a portable, modular concrete batching and mixing plant.
Further, in U.S. patent application Ser. No. 09/665891, filed Sep. 20, 2000, entitled High Volume Portable Concrete Batching and Mixing Plant, the inventors set forth a four-trailer modular and portable concrete batching plant. Simply stated, the compulsory mixer and the silo are mounted on separate trailers. The aggregate trailer and control trailer remain essentially unchanged.
In this disclosure, we cite the need for a compulsory mixer having capacity in the range of over 12 cyds (9.12 m3) of vibrated and compacted concrete. We have undertaken the design of such a compulsory mixer.
In this design, we have uncovered problems related to the discharge of such a large compulsory mixer. As it is understood that the discovery of a problem can constitute invention, the inventors claim invention both in the discovery of the problem to be solved as well as the solution to the discovered problem.
First, the reader will appreciate that a compulsory mixer having capacity in excess of 12 cyds is large and subject to high stress. Furthermore, as the length, depth and width of the mixing chamber increases, the volume of concrete contained in the static mixing chamber places considerable loading on the chamber. From empirical experience, there is an optimum ratio of width to length, as well as an optimum depth, when designing of a compulsory mixer to ensure the most efficient mixing. Ideally, the width and length of the mixer want to be approximately the same dimension. The height of the concrete in the mixer does not want to reach beyond the top of the mixing shafts. These become major design constraints when increasing the size of a mixer. Furthermore, in order to enable transport of the disclosed compulsory mixer, one is also constrained by a maximum practical and legal transport width of less than 12xe2x80x2 in North America and 3.5 m in Europe. Twelve cubic yards of concrete weighs in excess of 50,000 pounds. We have discovered that such loading on the eccentrically mounted elongate rectilinear discharge gate causes deflection in a traditional single gate design. Specifically, these gates are required to maintain a tight seal so that water, cement, and sand does not escape from the static mixing chamber. Unfortunately, as the length dimension of the gate increases, the tendency of the discharge gate to deflect also increases. Sealing would be an impossible task and unacceptable leakage would result.
It is important that the vertical gate deflection resulting from the weight of the gate and concrete be kept to a minimum in order for the gate seals to work effectively. This is especially important when considering how critical the proper water content is in a concrete mix. In our consideration of this design, it became apparent that if only one gate was used for this large (long) mixer that the cross-section of the gate would have to be increased substantially to keep this deflection to a minimum. Unfortunately, as the gate cross-section is increased the effective gate opening is decreased. A large gate opening is essential to achieve fast discharge for the short batch cycle times required for high production batch plants.
Second, normal measures to reduce deflection of the gate do not work. In the usual case, where a beam deflects under loading, adding to the depth of the beam normal to the loaded surface of the beam reduces deflection. This expedient will not work in the case of the eccentrically mounted elongate rectilinear discharge gate. Specifically, when the depth of the eccentrically mounted elongate rectilinear discharge gate is increased, the area available for discharge is correspondingly decreased. Stated in other terms, increased gate depth obstructs discharge, requiring longer intervals for the discharge. To prevent undue discharge delay, we have discovered that the design of the gate must be changed to prevent undue deflection.
Third, not only does the weight of the concrete deflect the eccentrically mounted elongate rectilinear discharge gate, it also deflects the static mixing chamber at the correspondingly elongated rectilinear discharge. Specifically, the dimension of the static mixing chamber changes relative to the gate. The tendency is for this rectilinear discharge opening to want to widen in the middle relative to the ends (a bulging effect). Again, unacceptable leakage occurs.
A compulsory mixer having a capacity in excess of 12 cubic yards has a static mixing chamber bottom defined from two horizontally disposed and interfering cylindrical shapes. A first cylindrical shape formed along a first horizontal axis defines a little over one-half the volume and profile of the bottom of the static mixing chamber. A second cylindrical shape formed along a second horizontal and parallel axis defines a little over a second one-half of the volume and remaining profile of the bottom of the static mixing chamber. The cylinders defining the bottom of the mixing chamber overlap or interfere at respective interfering sections interior of the volume of the mixing chamber. This interference occurs along cylindrical segments extending centrally of the volume of the static mixing chamber. Two centrally disposed elongate rectilinear discharge gates are each defined from an end in the bottom of the static mixing chamber to abut at the middle of the static mixing chamber. At the middle of the bottom of the static mixing chamber, a tensioning rod is placed extending normally across the abutted ends of the two rectilinear slots. Two eccentrically mounted elongate rectilinear gates are mounted for movement into and out of sealing relation to the rectilinear slot centrally of the static mixing chamber. These gates are mounted centrally of the mixing chamber to a pillow block and spherical bearing arrangement and actuated at the respective static mixer chamber ends by conventional piston drives. Reducing the gates in length reduces deflection. Further, the tensioning rod adjusts for chamber deflection. Finally, the eccentrically mounted elongate rectilinear gates can be rotated in opposite directions to deposit concrete on opposite sides of a concrete off-loading and elevating conveyor. Discharge of mixed concrete at maximally controlled rates can occur in a balanced fashion to an underlying conveyor for elevation and discharge from the large capacity compulsory mixer. The resultant compulsory mixer can be placed as the low profile foundation of a modular portable mixing plant.